MOLECULAR DYNAMICS BASED SIMULATIONS TO INVESTIGATE THE EFFECT OF RADIATION DAMAGE ON ZR AND ZR-NB ALLOYS

Abstract

Due to lower neutron absorption cross section, favourable thermo-mechanical and creep properties, Zr- Nb alloys are commonly used to manufacture pressure tubes and fuel claddings in nuclear reactors. Alloying with Nb helps in improving the corrosion and creep properties of Zr. In the course of lifecycle, nuclear materials undergo heavy irradiation, which eventually alters the response of the material to complex loading conditions in nuclear reactors. During collision cascades, high energy particles emanating from nuclear fission or fusion process interacts with the crystalline materials, which leads to formation of point defects or defect clusters. The presence of irradiation induced defects in conjunction with complex loading scenario acts as detrimental to the structural integrity of reactors. Although nuclear power plants are in use for many decades, but still the phenomenon of irradiation damage with respect to mechanical and fracture response of nuclear materials are in a premature state. Although, the period required to induce micro structural changes in nuclear materials spans from weeks to years, the primary irradiation damage events leading to such micro structural changes last only for a couple of picoseconds. Temporal and spatial scale involved in generation of irradiation induced defects, limits the experimental characterisation techniques to capture these phenomena. Hence, in order to characterize such atomistic scale phenomena, classical mechanics based Molecular dynamics (MD) and quantum mechanics based first principal calculations are emerging as viable alternatives to experimental techniques. In this thesis, MD based simulations were performed to study the effect of irradiation on single crystal, bi-crystalline Zr and Zr-Nb alloys. In order to capture the atomistic interactions between Nb-Nb, Zr-Zr, and Zr-Nb atoms, embedded atom method (EAM) and angular dependent potential (ADP) potentials with suitable parameters were employed. In this thesis, simulations were carried out to comprehend the effect of irradiation induced defects on the tensile and fracture properties of single crystals and bi-crystals of Zr, Nb and their alloys. Initially, single crystal Nb was explored for irradiation and tensile properties. The population of surviving point defects was inferred to be a function of simulation temperature, primary knock on atom (PKA) energy and direction of PKA. Tensile simulations of the irradiated samples revealed either clear channels or cross channelling of dislocations, which depends on the spatial distribution of irradiation induced defects. Simulations were also performed with single crystals of Nb and Zr containing centrally embedded crack subjected to opening mode of loading. It was predicted from the simulations that the governing mode of deformation as well as crack propagation depends on the orientation of the crack plane with principal slip planes of bcc Nb and hcp Zr. Twinning and dislocations were predicted to be the dominant mode of viii deformation in bcc Nb and hcp Zr, which further depends on the orientation of the crack plane with respect to principal planes in these materials. In next phase of the research work, efforts were made to apprehend the grain boundary energies and structure in Zr and Nb bi-crystals, and their effect on mechanical and fracture behaviour. Consecutively, the point defect formation energies were estimated with the help of molecular statics. A positive influence of grain boundaries was predicted from the simulations for irradiation induced defects. Presence of higher energy atoms in the grain boundary configurations helps in sinking point defects generated from irradiation induced defects. Additionally, effect of grain boundary configurations in Nb and Zr was studied with respect to tensile response. Later on, simulations performed with varying percentage constituent of Nb in Zr-Nb alloys revealed that 2.5% of Nb helps in reducing the point defect formation and vacancy migration energies in α-Zr bi-crystals. Pristine and irradiated Zr bi-crystals were also evaluated for their tensile strength and failure mechanism. Productive interaction of the moving dislocations with the point defects leads to hardening. The probability of this interaction was governed by both the number and spatial distribution of the defects in the bi-crystal. Cracks are inherently present in any real life polycrystalline material. In the final phase, interaction of cracks with the grain boundaries was probed in Zr and Zr-Nb bi-crystals. It was concluded that both orientation and distance of the crack from the grain boundaries affects the crack tip shielding. The grain boundary was capable of acting as a dislocation source or dislocation barrier depending upon its distance from the crack tip. In addition, grain boundary core structure and percentage of Nb precipitate also influenced the shielding properties and overall fracture behaviour of the bi-crystals.